An image display device for use in not only personal computers and workstations but also in portable information terminals generally have a display scrolling function. The display scrolling function is provided for moving an image displayed within a window display region when image data to be displayed can not fit the display or inside the window display.
On the other hand, the GIF (Graphics Interchange Format), which is one of standard image formats on the Internetwork, employs a compression/decompression system of a dictionary self-updating type using the LZW (Lempel Ziv Welch) system. In the compression/decompression system of a dictionary self-updating type adopted by the LZW system, the corresponding character string updates based on the entry number corresponding to the character string that has appeared in the past and was registered in the dictionary. The LZW system is disclosed in Japanese Examined Patent Publication No. 68893/1993 (Tokukohei 5-68893), and will be described in detail later under embodiments.
In this decompression system of the LZW system, it is always required to decompress the compressed image data from the beginning. Therefore, when an attempt is made to display in a rectangular portion, a part of the compressed image using the self-updating dictionary represented by the GIF, i.e., to display a rectangular portion of the image data file (image file) as extracted, if there is a sufficient memory capacity, only the rectangular portion as requested may be transferred to the video memory, etc., each time, after once decompressing the entire image.
However, devices with a small memory capacity such as a portable information terminal, adopt such partial rectangular display system that a portion in a compressed image file corresponding to a requested rectangle is decompressed on demand, thereby saving the memory. In this arrangement, by the described features of the compression/decompression system, the compressed image file is decompressed always from the beginning, and the decompression of the file corresponding to the requested rectangle is performed by searching a display start coordinate.
However, the described conventional image display devices have such a drawback that the closer is the portion to be decompressed to the end of the image file, a longer decompression time is required to reach the portion to be decompressed. For this reason, on the display screen, the closer is the rectangular portion as requested to the lower right corner of the display screen, the longer is the time required for reaching the rectangular portion as requested and displaying the image corresponding to the rectangular portion, which may be annoying for some users.